The Philadelphia Museum of Art has 'soft launched' a tagging application integrated into their on-line collections catalogue.

A tag cloud -- at http://www.philamuseum.org/collections/socialTagging.html -- shows tags assigned anonymously to the collection (tags don't seem to be linked to users though PMA has a My Museum / Gallery feature). Tags in the cloud are sized by frequency; the sequencing isn't immediately obvious.

Any user can add tags to a work, at the collection record. Tags can also be deleted by any user.

Tags are shown in the cloud immediately [presumably with some sort of filtering].

/jt